By default, sendmail will keep items in the queue for up to 5 days. If you want to make this something shorter, like 3 days, you can adjust the following in /etc/mail/sendmail.mc:
define(`confTO_QUEUERETURN', `3d')dnl
If you want to get super fancy, you can adjust the queue lifetime for messages with certain priorities:
define(`confTO_QUEUERETURN_NORMAL', `3d')dnl
define(`confTO_QUEUERETURN_URGENT', `5d')dnl
define(`confTO_QUEUERETURN_NONURGENT', `1d')dnl
